Why Do Orthodox Christians Call Mary the Theotokos?

One of the ways they expressed that was by saying the Virgin Mary, when she was pregnant, the child in her womb was already God.

Transcript:

You’ll find that Orthodox, very often they don’t say “Mary” or “Virgin Mary,” they say “Theotokos,” which is a Greek word, and it means “God bearer.” This was a very important title for her back in the early centuries, because the debate at that time was about, “Who is Christ?” Was Jesus Christ really the son of God? Or was he just a human being, that God blessed and turned into His Son—maybe at the baptism, he says, “This is my beloved Son.” Was He really one hundred percent God, or just partly God? Or was He one hundred percent God and not human at all? A lot of debates about that. So in those early centuries, in the middle of all that confusion, as the Church sorted out what it meant to say, “He is fully God, He is fully man at the same time.” One of the ways they expressed that was by saying the Virgin Mary, when she was pregnant, the child in her womb was already God. So to say she is the Theotokos, she is the God-bearer, is really to make the theological point about Christ, and about Him being God and the Son of God throughout all eternity, and in His earthly life from the moment of conception onwards.